1. Field of the Invention
The present invention relates to the field of radio communication apparatus such as mobile telephones, portable telephones, cordless telephones, and the like. More specifically, the present invention relates to a radio communication apparatus used in a radio communication system in which radio frequency signals are selectively transmitted in either analog or digital mode between a base station and a mobile unit.
2. Description of the Related Art
As the number of subscribers in cellular radio systems increase, it is desirable to incorporate digital signal transmission methods into the analog cellular systems that are presently available in order to transmit speech signals at a more efficient transmission rate. A system of this type is called a Dual Mode Cellular Radio System. In such a system, speech signals may be selectively transmitted between a base station and a mobile unit over speech radio links in either analog or digital mode. In addition, control signals are transmitted between the base station and the mobile unit over common control radio links.
The mobile units in such a system require circuits for transmission/reception in the analog mode and circuits for transmission/reception in the digital mode. The result is that the number of components required for the mobile unit is increased, thereby increasing the cost of manufacturing the mobile unit. This is especially true when an echo-canceller is employed to cancel echo signals caused by acoustic coupling between a microphone and a loudspeaker, for example, during hands-free operation. As a result, two echo-cancellers are needed: one for digital mode operations and one for analog mode operations.